superconductivity

Batch and CrO2 (Streatham)

coffee in Streatham

Batch & Co, Streatham Hill

A short while ago, on the advice of London’s Best Coffee (and Beanthere.at), I headed along to Streatham to try a couple of cafés including Batch & Co along Streatham Hill Road. The café is quite modern and cubic with plenty of tables at which to sit and enjoy some good coffee and food. Another interesting recommendation from these sites to add to the list. The counter is on the left as you enter and there was a good selection of cakes on offer that day. Is it possible to have too much cake in one day? Sadly, possibly it is and so, as I had already had my fill of cake at a previous café, I kept with just an Americano (roasted by Caravan). Tap water (infused with mint) was available at each table which was greatly appreciated on such a hot day as the one on which we visited.

There were many things to notice in Batch and Co. The street/bus sign above the counter, the large selection of books in the corner (what a shame the seats next to the shelves had been occupied already!), the corrugated zinc walls and then, the cassette tapes on the tables. What a blast from the past. Sadly these tapes were no longer being used to store music but instead as table number indicators. Now ordinarily, I think these cafe-physics reviews should be the sort of science that is accessible to everybody, the sort of observation that anyone could make. But today, today the temptation is just too great, because these cassette tapes are linked to something that is being researched in an obscure but very novel effect that just happens to be an area of research for me. So today, I hope you will stay with me as I take you from Batch & Co to a very odd effect that happens when things (cassette tapes) get very cold.

coffee and cassette tape in Batch and Co

Coffee and tape. Who knew how special the tape material would be?

Those cassette tapes used to work by writing and reading magnetic information. So the actual tape bit needs to be a magnetic material. The first generation of tapes were made with ferric oxide (Fe2O3) but later, and seemingly better, music tapes used chromium dioxide, CrO2, as the tape material. Nowadays the technology of tape cassettes has been superseded by other media but the material CrO2 lives on, it turns out it is a very odd type of material.

Just like iron, chromium dioxide is magnetic, which is why it was used in tapes. But chromium dioxide is a very special type of magnet in that it is what is known as a fully spin polarised magnetic material. To understand what that means, it’s helpful to compare it with iron or copper or indeed, any other metallic material that you can think of. Metals conduct electricity because the electrons in them are free to move from one contact to another and hence carry a current. Electrons are negatively charged particles but they also have a property called “spin”. Although spin is associated with angular momentum (rotation), it is fundamentally a quantum mechanical property of subatomic particles and so shouldn’t be thought of as being about the electron’s rotation on its axis (rather like the Earth rotates). Indeed, it seems that this quantum mechanical property of “spin” is something that is very hard to pin down, even amongst physicists (see here). So instead, generally speaking, we just think about spin having two ‘directions’: spin up and spin down.

tape supporting a table, Batch and Co

An alternative use for a cassette tape. Poor tape.

Ordinarily, the electron spin doesn’t have that much effect on how much current the metal can carry (its ‘resistance’). Indeed for most metals, the number of spin up electrons is roughly equal to the spin down ones. However this is not true of chromium dioxide. Although it is a metal, all of the electrons that conduct the electricity through it are of one spin type. All the electrons are either ‘spin up’ or they are all ‘spin down’. This is spin polarisation. It is something that could never happen in copper.

There are many reasons that this could be interesting, both technologically and purely from the perspective of it being quite beautiful physics. What turns it from interesting to a really big question though is what happens when chromium dioxide interacts with another set of materials, superconductors.

Superconductors are materials that can carry large amounts of current with zero electrical resistance. This property makes them great for things like MRI machines in hospitals where large magnetic fields require the sort of currents superconductors can carry easily. How they are able to do this gets a bit complicated but what is crucial for this subject is the fact that to conduct a supercurrent they need to have zero spin polarisation: they need to have equal numbers of spin up and spin down electrons. (If you are interested in how superconductors superconduct you can read more about them here and here).

cassette tape at Batch and Co

Who knew that this tape was so special?

Now imagine, you have a wire of a superconductor such as very cold niobium (all spins are equal) that you connect to a wire (or a tape) of chromium dioxide (only one spin possible). You may think that if you tried to pass an electrical current down that connection there would be a problem. And you would be right: To conduct electricity, there have to be equal numbers of spin up and spin down electrons on the superconductor side but only one spin type can get through to the chromium dioxide side. There would be an electrical traffic jam. Which is all very logical and reasonable but it isn’t what happens. Instead, for reasons that we still do not understand, not only does the electrical current get through the connection, the chromium dioxide itself becomes superconducting through its proximity to the superconductor. By itself it could never superconduct but somehow, the superconductivity is leaking¹ into the chromium dioxide at the joint between the superconducting wire and the chromium dioxide tape. And it shouldn’t do this because everything we understand about superconductivity requires there to be electron pairs of spin up and spin down and everything we understand about chromium dioxide tells us that is absolutely not the case.

So how does it work? Surely these two effects (of superconductivity and spin polarisation) are incompatible with each other? Is there something peculiar about chromium dioxide that makes it so susceptible to this strange effect? We do not yet know (though we have a few ideas). Many groups around the world are looking at this odd effect including a network of universities in the UK. It is taking us a lot of research and quite a few meetings involving coffee to work it out but hopefully one day we’ll get there.

In the meantime, it may be worth pondering just how special those cassette tapes really were.

Batch&Co is at 54 Streatham Hill Road

¹Yes, “leaking” is, perhaps surprisingly, one of the technical words for what happens in the proximity effect.

 

Super cold brew

Cold brew coffee with ice

Cold brew coffee served with ice. Image from pixabay.com

How cold do you drink your cold brew? Poured over ice? As an experimental physicist who works with liquid nitrogen (& helium), I was initially quite intrigued to learn of nitro cold brew coffee. Could it be coffee that somehow uses liquid nitrogen to fast-cool it, what would that do to the taste? You would expect liquid nitrogen (at -196ºC) to rapidly cool the coffee below its freezing point, after all, it is how Heston Blumenthal makes ice cream. To make a drink-able cold-brew with liquid nitrogen would require great skill, especially given the potential health risks. It would be another situation where you may well ask yourself, “what’s the point?”

However, it turned out that the reality was far more mundane, gaseous nitrogen is passed through cold brew coffee to create a drink with a silky mouthfeel. A smooth drink that comes straight from the tap just like stout. Such a drink is going to behave as an ordinary liquid and chilled only to the point that it is kept in the vat. The novelty would presumably come from the mouthfeel introduced by the many tiny bubbles distributed through the drink. Just as with water, if you cooled the nitro-brew below its freezing point it would solidify and form coffee cubes. No real difference to get excited about. But what if there was a very different sort of liquid, a “super liquid”, that didn’t behave like water, coffee or even liquid nitrogen but one that could leak through solid cups?

Superfluid helium is such a liquid. Like water, oil or even liquid nitrogen, when you cool helium (the same gas that is in party balloons)∗, it becomes an ordinary (but very cold) liquid at -269ºC. But unlike those liquids, when you cool it further, below -271ºC, it does something very odd indeed. It becomes a superfluid in which the liquid moves with zero friction or equivalently, zero viscosity (honey is very viscous, water is very much less so).  And it is because of these properties that it can do some astonishing things such as stream through cracks in containers that were thought impermeable (see the video at 0:52m), or even climb the walls of the container it is in (1:13m)!

 

To explain the behaviour of superfluid helium it is necessary to use quantum mechanics. Indeed, Fritz London (1900-1954) is said to have described both superfluidity and superconductivity (which happens in solids) as “quantum mechanisms on a macroscopic scale”. At the heart of the theory of superfluidity is the idea that the helium atoms fall into the lowest energy ground state possible, a Bose-Einstein condensate. To form a Bose-Einstein condensate, the particles (atoms of helium) have to  be bosons rather than fermions. All particles in nature can be categorised as either bosons or fermions. The difference between the two types comes from another quantum property of particles, the spin. Spin is related to the angular momentum of the particles and, this being quantum mechanics, can take only discrete values, either whole number or half integer numbers.

cold brew, doublemacbex

Another photo of cold brew coffee, this time from Bex Walton (flickr) – note the condensation around the rim, much could be said about that. Image CC licensed.

Bosons are particles with integer values for spin, fermions are particles with half integer values. Most of the elementary particles you will have heard of are fermions: electrons, protons, neutrons, they’re all fermions. Some particles however, such as the photon (the particle of light) are bosons. Helium 4 atoms are effectively composite bosons, because of the combination of 2 protons, 2 neutrons and 2 electrons that make up the atom. When you add their individual (half-integer) spins, you will get an integer spin, hence a boson not a fermion. The distinction is important because while bosons can share a lowest energy state (the Bose-Einstein condensate), fermions cannot. Quantum mechanically, no two identical fermions can share an energy level (the Pauli exclusion principle), so you can never get to a state where all the fermions are in the lowest energy state. There are practical, every day consequences of this for us, such as the way metals such as copper conduct electricity and heat, the fact that the electrons in the metal are fermions turns out to be crucial for us to understand how metals ‘work’. In contrast, the fact that the helium atoms are in the lowest energy state in super-fluid helium means that the ‘liquid’ behaves very strangely indeed.

We seem to have come a long way from the idea of a cold coffee. But perhaps next time, if someone offers you a “super cold brew” take a moment to think of the physicists who get to play with some real super cold superfluids†. Hope you enjoy the video.

 

*Technically it is Helium 4 that becomes superfluid at 2.2 K (-271ºC). The rarer isotope, Helium 3, does not become superfluid until much lower temperatures and even then, the superfluidity has some very special properties.

†Although I do get to work with liquid helium (and although it is mostly helium 4), I work at the relatively ‘hot’ temperatures at about -269C. At this temperature the interest is not so much in the liquid helium itself but its use as a coolant for other materials.